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NEWS & VIEWS
NATURE|Vol 452|6 March 2008
QUANTUM PHYSICS
Tangled memories
Lene Vestergaard Hau
The latest quantum trick — mapping two entangled photon states onto two
separate regions of an atomic cloud, and then retrieving them — could be a
fillip for applications, among them quantum cryptography.
On page 67 of this issue, Choi et al.1 recount
how they store two ‘entangled’ photon states in
a memory consisting of a cloud of cold atoms,
and then, after a certain delay, retrieve those
self-same states from the cloud. The optical
modes are stored in spatially separated regions
of a single atom cloud, but there is no reason
why the technique should not be used to
imprint the same quantum states on two distinct atom clouds separated by a macroscopic
distance. That would allow the controlled
entanglement of two distant atomic samples
— a step that might be of great importance
for the practical implementation of quantum
protocols to generate secure keys for the transfer of information over public networks.
Choi and colleagues’ experiments start off
with a light pulse containing a single photon.
Like most of us, this photon soon comes to a
point in life — in its case, a beam splitter — at
which it is faced with a choice between two
paths. In such a situation, the quantum world
is kinder than the classical: it allows the photon to take both paths at the same time. If we
set up a light detector to monitor one of the
two paths, we will register a click (a photon hit)
or no click, each with a 50% chance. But say
some other (possibly distant) observer sets up
a detector to monitor the other path. In this
case, if this second observer detects a click, that
instantaneously affects our own measurement:
we will detect ‘no click’ with 100% probability
(or vice versa, an absolutely certain click if the
remote observer has detected no click).
This is entanglement: the strange correlation of two spatially separated quantum states.
Entangled states are hard to maintain, because
interactions with the environment destroy
the entanglement. As a result, they are rare in
our macroscopic world. But that doesn’t stop
them being essential to quantum information
processing, for computing, teleportation and
encryption applications.
In a classical computer, information is stored
in strings of bits of value 0 or 1. The quantum
bits of a quantum computer, on the other hand,
can be in ‘superposition states’: they can be 0
and 1 at the same time. Here, entanglement
comes into its own. Let’s say the quantum computer is set up to calculate the output value of a
function that varies periodically with its input
value (a sine wave, for example); we wish to
find that period. The quantum computation
leaves the system in a superposition of matched
pairs, in which the output register holds the
function value for the corresponding input.
The two registers are in fact entangled: a
measurement of the output register will immediately affect the input register. For a periodic
function, in which many input values correspond to a particular output value, the input
register ends up in a superposition of precisely
these inputs. After just one run-through of
the calculation, therefore, global information
Atomic gas
Phase-shifter
Mirror
Control
beam
Beam
splitter
“A three-dimensional model of
the myoglobin molecule obtained
by X-ray analysis.” By Drs J. C.
Kendrew et al. — Until five years
ago, no one knew how, in practice,
the complete structure of a
crystalline protein might be found
by X-rays, and it was realized
that the methods then in vogue
among protein crystallographers
could at best give the most
sketchy indications about the
structure of the molecule. This
situation was transformed by the
discovery, made by Perutz and
his colleagues, that heavy atoms
could be attached to protein
molecules in specific sites and
that the resulting complexes gave
diffraction patterns sufficiently
different from normal to enable
a classical method of structure
analysis, the so-called ‘method
of isomorphous replacement’, to
be used to determine the relative
phases of the reflexions … The
present article describes the
application, at low resolution, of
the isomorphous replacement
method in three dimensions to
type A crystals of sperm whale
myoglobin. The result is a threedimensional Fourier, or electrondensity, map of the unit cell,
which for the first time reveals
the general nature of the tertiary
structure of a protein molecule
… Perhaps the most remarkable
features of the molecule are
its complexity and its lack of
symmetry. The arrangement …
is more complicated than has
been predicated by any theory of
protein structure.
From Nature 8 March 1958
100 YEARS AGO
Constructive/
destructive interference
Decelerating
Slowed
Accelerating
Figure 1 | Light split and slowed. In Choi and colleagues’ experimental apparatus1, a light pulse
(red arrow) is split into two entangled states and fed into an atomic gas. There, under the influence of
controlling laser beams, the split pulse is slowed, and the two entangled quantum states are imprinted
onto two separate regions of the gas — effectively creating entanglement between the atoms of these
regions. The light is then extinguished, and after a delay the photonic quantum states are regenerated
from the information stored in the atoms. The amount of remaining entanglement is measured through
the degree of constructive or destructive interference when the regenerated optical fields are recombined.
It is reported by The Hague
correspondent of the Globe
(March 3) that Prof. Kamerlingh
Onnes, professor of physics in
the University of Leyden, has
succeeded in liquefying helium.
ALSO:
In the report of the Maidstone
Museum, Library, and Art
Gallery for 1907, attention is
directed to the unprecedentedly
large number of visitors during
the year.
From Nature 5 March 1908.
50 & 100 YEARS AGO
50 YEARS AGO
37
NEWS & VIEWS
about the function — its period — is contained
in the input register. After some ‘fiddling’, this
period can be read out with many fewer operations than a classical computer requires2. (In a
classical calculation we would have to run the
computation many times, once for each input
value, to slowly, step by step, build up global
information about the function.)
Encoding this information in material
quantum states, such as those of atomic clouds,
is a particularly promising avenue towards
implementing quantum-computational
schemes: the interactions between atoms can
be strong, and processing (through controlled
changes to the quantum states) can happen
fast. Long-distance transmission of the resulting output quantum states, on the other hand,
works better with light: light can travel fast and
with minimal losses in optical fibres. Reliable
and efficient ways of mapping quantum information between light and matter are therefore an essential component of any quantum
information network.
In their experiments, Choi et al.1 achieve
this transfer using ‘slow light’3. Under particular conditions dictated by quantum mechanics, the speed of a light pulse that has been
injected into an atom cloud illuminated by a
control laser can be reduced by many orders of
magnitude. As the pulse slows, its spatial extent
shrinks, and it ultimately fits snugly inside the
atom cloud (Fig. 1). The pulse affects the internal states of atoms within its localized region,
creating a hologram-like imprint of itself in the
atom cloud. If the control laser is turned off,
the light pulse is halted and extinguished, but
its atomic imprint remains in the cloud. If the
control laser is turned back on, the same process runs backwards: the light pulse is regenerated and moves on, exits the atom cloud and
speeds back up. Such storage of optical information has been demonstrated for classical
light pulses containing many photons4,5 and
for single-photon light pulses6,7.
The authors show that the entanglement of
two light fields also survives such storage. They
create their single-photon pulse by generating
a single atomic excitation in a separate ‘source’
atom cloud, using the slow-light effect to read
it out. The entangled optical modes generated
at the beam splitter are injected into the main
atomic sample, separated by 1 millimetre. After
a microsecond, the authors are able to regenerate the optical fields. They then introduce
a phase shift into one before the two modes
recombine at a second beam splitter. Depending on the phase shift introduced, constructive
or destructive quantum interference between
the two optical paths is measured at a detector
after the second beam splitter. The contrast of
the fringes obtained as the phase is varied tells
us the quality of the entanglement.
So what is the next step? Separate control of
the phase relationship between the two optical
modes’ input to the atom cloud would allow the
controlled storage and revival of actual quantum bits. Choi et al.1 use cold atoms for their
38
NATURE|Vol 452|6 March 2008
experiments, laser cooled to about 125 microkelvin above absolute zero to avoid thermal
smearing of the stored holographic imprints;
but it would be interesting to cool things down
further, forming the phase-coherent state
of atomic matter known as a Bose–Einstein
condensate, which offers much-increased
possibilities for storage and controlled processing8. The storage of entangled optical fields in
two separated atom clouds also needs to be
formally demonstrated. Such entanglement
would allow the efficient sharing of quantum
keys for secure encryption9, and the transfer
(teleportation) of quantum bits between two
atomic clouds by simple transmission of pairs
of classical bits10.
Will any or all of this ever get to a stage at
which it can be used in practical devices? That
remains to be seen. We can state, however,
that a century after quantum mechanics was
discovered, the possibilities it offers continue
to boggle our minds.
■
Lene Vestergaard Hau is in the Lyman Laboratory,
Physics Department, Harvard University,
17 Oxford Street, Cambridge, Massachusetts
02138, USA.
e-mail: [email protected]
1. Choi, K. S., Deng, H., Laurat, J. & Kimble, H. J. Nature 452,
67–71 (2008).
2. Shor, P. W. in Proc. 35th Annu. Symp. Found. Comput. Sci.
(ed. Goldwasser, S.) 124–134 (IEEE Computer Soc. Press,
Los Alamitos, CA, 1994).
3. Hau, L. V. Sci. Am. 285 (1), 66–73 (2001).
4. Liu, C., Dutton, Z., Behroozi, C. H. & Hau, L. V. Nature 409,
490–493 (2001).
5. Phillips, D. F., Fleischhauer, A., Mair, A., Walsworth, R. L. &
Lukin, M. D. Phys. Rev. Lett. 86, 783–786 (2001).
6. Chanelière, T. et al. Nature 438, 833–836 (2005).
7. Eisaman, M. D. et al. Nature 438, 837–841 (2005).
8. Ginsberg, N. S., Garner, S. R. & Hau, L. V. Nature 445,
623–626 (2007).
9. Ekert, A. Phys. Rev. Lett. 67, 661–663 (1991).
10. Bennett, C. H. et al. Phys. Rev. Lett. 70, 1895–1899 (1993).
NEUROSCIENCE
A complex in psychosis
Solomon H. Snyder
The molecular basis of psychoses such as schizophrenia remains largely
mysterious. The interaction between two of the brain receptors involved
adds to evidence that will help in the search for explanations.
This is a story that involves three types of
receptor in the brain that influence human
perception and behaviour (those for the neurotransmitters dopamine, serotonin and glutamate), and the drugs that block or enhance their
activity. Such drugs are used by researchers to
investigate the causes of psychotic disorders
such as schizophrenia, and by clinicians to treat
patients. Classical antipsychotic drugs, designated ‘typical neuroleptics’, act predominantly
by blocking dopamine D2 receptors. A new
Potential
antipsychotics
LY341495
LY379268
LY2140023
mGluR2
Hallucinogens Antipsychotics
(LSD, DOI,
(Atypical
mescaline)
neuroleptics)
2AR
Psychotomimetic or antipsychotic actions
Agonist:
stimulatory effect
generation of more effective ‘atypical neuroleptics’ also blocks a subtype of serotonin receptor
known as 5-HT2A, or more simply as 2AR. And
last year we had the promising report1 of a drug
that mimics the effect of glutamate at a subtype
of one of its receptors, metabotropic glutamate
receptor 2 (mGluR2), and that seems to be as
effective as atypical neuroleptics.
This is the background against which the
paper by González-Maeso et al.2 appears
(page 93 of this issue): these authors show that
Antagonist:
inhibitory effect
Figure 1 | Receptor interaction.
González-Maeso et al.2 find that the
metabotropic glutamate receptor 2
(mGluR2) and serotonin 5-HT2A
receptor (2AR) physiologically bind
each other, leading to reciprocal
regulation of their functions.
Agonists that stimulate mGluR2
are antipsychotic, whereas 2AR
agonists, such as hallucinogens, have
the opposite effect. It is conceivable
that the clinically significant antischizophrenic effects of LY2140023,
an mGluR2 agonist1, derive from
reducing the excessive — and
hence hallucinogen-like — activity
of 2AR. DOI, 2,5-dimethoxy-4iodoamphetamine.